Basement insulation is often overlooked in an older home's overall energy performance. If attics are the crown on top of houses, then foundations are the thrones on which houses sit. Except in the mildest climates, insulation should always be installed under slabs, at slab perimeters, and at basement and crawl space walls.
Actually, as far as under the slab is concerned, I would say to keep that to a minimum (use mainly vapor barrier), because you want to couple the earth and the house to exchange heat freely.
That way, in the winter, when the ground temperature is temperate below the frost layer (about 50 degrees Farenheit), you direct that "heat" upward (not outward through the side foundation walls) into your foundation and by extension, your house. This preheats or precools your home so your heating or cooling systems don't work too hard, or depending on where you are, at all. The reverse is true for the summer months. That cool 50 degrees is keeping your house cooler than the outside air (which is why the basement is the place to be in the hot months). This is the primary concept behind geothermal heat.
There is one big difference between foundations and roofs: your foundation is buffered by the ground's temperature, so it doesn't experience the radical temperature swings that the roof does. Another difference is that foundations are usually harder and more expensive to work on after the fact, especially if they require leveling or re-pouring, which may require the house to be jacked up or lifted off the slab.
How Much Basement Insulation Do You Need?
Remember that concrete has an piddly R-Value of 0.5, so, you might as well ignore its contribution. The R-Value is measured in inches; if you have a 6 inch thick concrete foundation, its total R-value is 6 inches x 0.5 = 3. In other words, to calculate R-values, obtain the thickness of the material, and its R-Value per inch, and multiply the R-Value by the total thickness of the material. If you have several materials next to each other for insulation, calculate the total R-Values of each, and add them all together.
Depending on where you live, how much of the work you are willing to do yourself, and the age of your house, basement insulation can pay for itself in savings from 6 months to 20 years. The less insulation your house has now, the faster the payback when you upgrade. The only regions that this is not true are in warm climates.
Use the insulation calculator below to determine how much insulation you need in each part of your house (not just the foundation). The report is based on your location (for U.S. residents only). Before you get your report, check out the tell-all Home Insulation Diagram in PDF format that shows what areas of a house actually need insulation.
Your Frost Line and Basement Insulation
The deeper your foundation goes, and the more foundation insulation around the perimeter, the more it "tempers" your home. Ideally, your foundation should be built about a foot or more below your frost line.
If your foundation was built without perimeter insulation, it can be added on after the fact, but it will involve digging/excavating around the exterior. 2" foam boards on the exterior are very effective at reducing heating and cooling losses. It's burdensome, but worthwhile.
For a fantastic demonstration of what you can do to insulate your foundation, read Guy Marsden's account of insulating a concrete foundation wall . Guy shows in detail what he did to his Maine home, and clearly documents the temperatures before and after his perimeter insulation.
Below are two photos from Guy's website that show the temperature on the outside of the walls after insulation. The colder temperature (blue color) on the exterior wall shows that there is no heat leaking to the outside. Note the warm temperature (yellow) at the bottom of the foundation showing heat leaking below the insulated area which goes 8" below ground.
To sum it up, using basic materials and techniques, he was able to keep the interior foundation walls an average of 11 degrees F warmer in Maine in the coldest months without any additional heating - that's a lot of heating dollars saved. Very impressive. Way to go, Guy!
By the way, you can use the same foundation wall repair techniques for interior basement insulation. If you are going down to the bear concrete walls, you can fasten foam sheets, and then finish off your basement walls. And finally, if you do this for both interior and exterior sides of the foundation wall (sandwiching the concrete between the insulation), you are mimicking ICF (insulated concrete form) foundations and reaping the benefits for as far down as you can dig (10-12 inches) below the surface on the outside.
Next, go to Concrete Foundation Waterproofing - presented from the standpoint of energy efficiency, do this before insulating. Waterproofing is like moisturizer for your skin: no one sees it, but you know you're protected from the elements AND you are preventing wrinkles!
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